The aforementioned copending application refers to the commonly owned U.S. Pat. Nos. 3,799,199, 3,975,128, 3,706,515, 3,954,926 and 4,226,543 and other publications mentioned in that application for background in systems for bringing about the mixture of two or more reactive components in a mixing chamber prior to or concurrently with the expulsion of said materials into another vessel, e.g. a mold. In the production of polyurethane, for example, a di-isocyanate or polyisocyanate component is reacted with a diol or polyol component to produce the settable reaction mixture by separately feeding these components into a mixing chamber and displacing the intimately formed mixture from the chamber into a mold in which the mixture can be set. Such mixing heads may also be used in the dispensing of the reactive mixture for other purposes as well and, in general, for any system requiring the combination of two or more components to form a reaction mixture.
In general, the conditions under which two components are mixed require that the two components be held separate from one another until the instant at which they enter the mixing chamber, since any premature contact of the two components with one another will result in setting. Such premature contact may result in the reaction of the two components to form a hardenable mass obstructing further outflow of one or both of the components of the mixture. In order to prevent obstruction of the passages, both components are generally provided in a highly flowable form and are circulated by pumps or the like through the control plunger, piston or ram in one position of the latter, entering the mixing chamber only after retraction of this control member into another position.
It is also important, to avoid the need to clear out solidified products and to permit reuse of the head, to provide the plunger, piston or ram so that it completely expels the reaction mixture from the mixing chamber. It has already been noted that this mixing chamber can open directly into a mold or other vessel.
It has been found, in some cases, to be advantageous to discharge this chamber into a duct leading to a mold and at substantially right angles to the mixing chamber, this duct being provided in turn with a second piston, plunger or ram to drive the mixture out of the channel. Particular reference may be had to U.S. Pat. No. 3,975,128 and the German Pat. No. 2,327,269 in this regard.
Both of these publications deal with a high pressure mixing head in which the additional channel has the effect of quieting the highly turbulent mixture driven from the mixing chamber into this channel. The quieting channel is comparatively narrow and long and the second piston serves to clear the channel at the end of each mixing phase and thus prevents the channel from being plugged up by the reaction mixture. It is known in other mixing heads to control the pressure in the mixing chamber and to regulate the precise metering or reaction of the material, especially with small dispensed quantities per operation (e.g. several grams per cycle), by providing movable displacement bodies in the mixing chamber itself (see German Pat. No. 2,065,841 and Printed German Patent Application-Auslegeschrift No. 2,612,812) downstream of the mixing chamber in the discharge passage (see Printed German Application-Auslegeschrift No. 2,544,749). These systems, however, have various disadvantages. For example, some of them may be excessively space consuming at large for many purposes, and can be expensive and difficult to handle. Others are difficult to control and control systems for them may be expensive. Still others cannot be readily repaired or maintained.
Reference may be had to German Open Application-Offenlegungsschrift No. 3,040,922 which describes a mixing head in which the cleaning plunger has a retracted position in which the mixing chamber outlet opening is partially covered and which thus establishes an adjustable throttle for the reaction mixture leaving the mixing chamber. This arrangement also has a disadvantage, namely that in the region ahead of the mixing chamber outlet at the cleaning plunger, residues of the mixture can accumulate which can harden to a greater or lesser extent and can be entrained directly or subsequently into the mold cavity and thereby detrimentally affect the quality of the product.
This problem was also recognized in the aforementioned application which provided a modification of the system of U.S. Pat. No. 3,975,128, so that directly at the mixing chamber outlet opening, a filler body or cleaning plunger is provided which partly obstructs the outlet opening of the mixing chamber in the discharge phase, i.e. while the mixture is forced by the first plunger from the mixing chamber into the channel at right angles thereto. In this approach, a stop is provided for the second piston (cleaning plunger) which is adjustable to control the degree of obstruction of the mixing chamber opening and to hold the piston so that a predetermined amount of the free end of the piston or plunger projects across the mixing chamber opening or such that the obstructing body lies at a preferably adjustable location downstream of the mixing chamber opening.
The second piston or plunger has, in addition to its final or cleaning function, the additional function of serving as a filler body to partially fill the flow passage for the mixture and to constitute a body of predetermined but adjustable volume at this location.
This arrangement eliminates the need for a separate filling body and the means for positioning a separate filling body or adjusting the position of the separate filling body.
In that system, moreover, the obstructing body has an adjustable stop which can limit the displacement of the second piston in its retracted position. This adjustable stop can be a screw or the like and provides extremely simple means for setting the throttle in position of the obstructing body.
The second piston is displaceable by fluid pressure between its forward and retracted positions and the abutment is so arranged that it engages directly a fluid pressurizable surface of the second piston.
According to yet another feature of the earlier application, the second piston and its actuator has between them a device provided for the selective displacement of the second piston relative to the actuating member so that the maximum stroke of the second piston is substantially equal to the length and the direction of the stroke of a mixing chamber outlet opening. This adjustment means between the second piston and the working piston can be a hydraulically activated device working against the force of a spring with the force being effective in the direction of the forward position of the second piston.
Furthermore, between the second piston and the abutment of the actuating member, i.e. the working piston, a stroke is provided which can be equal to the displacement of the second piston, the latter being formed with a lifting piston pressurizable by the medium in the direction of the retracted position of the second piston or plunger.
This latter means enables, in spite of the setting of a predetermined degree of obstruction of the mixing chamber outlet opening, a retraction of the second piston or plunger so that it can completely clear this opening, thereby allowing mixture residues to be completely stripped from the system and even from the surfaces of the member acting as the obstruction.
In the process described in that application, the mixing phase finds both of the pistons retracted, the first or mixing piston being fully retracted and the second or control piston being partially retracted so that its leading projects beyond the boundary of the mixing chamber opening and forms an obstruction as previously described. At the end of the mixing phase, the first piston is advanced to drive the mixture out of the mixing chamber and upon reaching the end of the mouth or opening thereof the second piston is retracted slightly for stripping of the residues and then is advanced fully to drive the mixture out of the channel.
Consequently, in the system of the earlier application the disadvantage described previously is eliminated by providing between the cleaning plunger and its working piston, a selectively actuatable arrangement with the aid of which the throttle of the cleaning plunger can be relieved or nullified.